get.chr.lens: Get chromosome lengths from build database

Description

Quick and easy way to retrieve human chromosome lengths. Can select from hg18/hg19 (ie,
build 36/37), or any future builds (hg20, etc) stored in the same location on the build website.
Default is to return lengths for 22 autosomes, but can also retrieve X,Y
and Mitochondrial DNA lengths by 'autosomes=FALSE' or n=1:25. Even if not connected to
the internet can retrieve hard coded lengths for hg18 or hg19.